This invention is in the field of gas atmospheres employed in furnaces; more particularly, the invention relates to the generation of a carrier gas for use in carburizing furnaces.
The Metals Handbook, Eighth Edition, Volume 2, Heat Treating, Cleaning and Finishing, prepared under the direction of the ASM Handbook Committee and published by the American Societies for Metals, Metals Park, Ohio, at page 94, column 2, reports on carrier gases used in gas carburizing furnaces. In particular, carrier gas Class 302 is produced by the endothermic reaction of natural gas and air and has a composition by percent volume of 39.8% nitrogen, 20.7% carbon monoxide, 38.7% hydrogen and 0.8% methane. The text goes onto note:
"Endothermic gas (Class 302) is generally the preferred type of carrier gas for use in gas carburizing furnaces and is the most widely used. It offers a broad range of carbon control, a moderate amount of carbon availability for carburizing, and when operated with dew points of +20.degree. F. and above, continuous operation without weekend shutdowns for burn out."
Class 302 type carrier gas is a preferred and popularly used carrier gas in carburizing processes. Many control schemes for carburizing now require a carrier gas having the composition of Class 302. Carrier gases having this composition are presently generated from natural gas or methane. Should the availability of methane or natural gas be restricted, it will be necessary to find a suitable replacement which can be used without requiring changes in the method of carburizing control. Preferably the replacement should have a composition as close to that of Class 302 type carrier gas as possible.
The present invention is a method of producing Class 302 type carrier gas from methanol and nitrogen. It will be helpful in understanding the method of the present invention to consider the state of the art.
U.S. Pat. No. 3,519,257 describes a process and apparatus in which the novelty resides in the use of an annealing furnace which has a catalytically active surface therein for use in the in situ formation of gases. The walls of the furnace are treated with a catalyst. Suitable gases including natural gas, methane, etc., or liquid hydrocarbons, such as alcohol may be reacted with oxygen in the presence of the catalyst to form a suitable atmosphere within the furnace to carburize the surface of metal.
U.S. Pat. No. 3,620,518 is a continuation-in-part of U.S. Pat. No. 3,519,257 with the additional step of preheating oxygen containing gas, such as air, before feeding it into the furnace.
Both of these patents describe a process for producing both a carrier gas and carburizing gas in situ by catalytically reacting feed gases with air inside the annealing furnace. Neither patent indicates a particular method for producing the popular Class 302 type carrier gas or discloses, describes or suggests a suitable control system for making a Class 302 type carrier gas in situ in the furnace.
U.S. Pat. No. 3,201,290 describes a process for automatically controlled carburizing of the surface layer of steel articles by gas carburizing in a gas atmosphere formed in the furnace chamber by thermal decomposition of separately fed substances on which yields a carrier gas and another which yields a carburizing gas. The combination of feed materials being such that during the cracking and carburizing reactions, the resulting two gases provides substantially the same and substantially constant gas composition. The supply of the gas providing substances, preferably of the substance yielding the carburizing gas, is controlled by continuously determining the content of one constituent of the furnace gases. Methanol is described as suitable liquid which may be cracked in the furnace to form a carrier gas of carbon monoxide and hydrogen. The methanol cracking occurs at a furnace temperature of 800.degree. C. (1472.degree. F.). The reaction products consist mainly of 1/3 carbon monoxide and 2/3 of hydrogen. However, if the carburizing liquids are added to this mixture according to the carbon demand of the surfaces, the carbon monoxide content changes very differently for different carburizers with increasing carburizing gas consumption. The carburizing process must be carried out under conditions which are suitable for the methanol to crack in order for this process to succeed. Additionally, the methanol cracking reaction is affected by the carbon demand of the metal surfaces and the parallel formation of carburizing gases within the furnace chamber. The use of methanol as a source of carrier gas in this process is successful only under very particular conditions and is affected by other occurrences within the furnace chamber. Although carbon monoxide and hydrogen are formed for uses as a carrier gas in this process, they do not necessarily result in a carrier gas of the type of Class 302.
Where methanol has been used in carburizing furnaces in the past, it has been injected directly into the furnace chamber. The methanol in the furnace chamber either reacts with other materials or cracks to form suitable carrier and carburizing gases. Nowhere has the reaction of methanol within carburizing furnaces been directed toward the production of the preferred Class 302 type carburizing gas inside or outside the furnace chamber, nor has a control system been disclosed which can control reaction of methanol within the furnace chamber to yield Class 302 type carburizing gas.